Facile magnetic biochar production route with new goethite nanoparticle precursor

Journal Pre-proof Facile magnetic biochar production route with new goethite nanoparticle precursor

Divine Damertey Sewu, Hai Nguyen Tran, Godfred OhemengBoahen, Seung Han Woo PII:

S0048-9697(20)30601-X

DOI:

https://doi.org/10.1016/j.scitotenv.2020.137091

Reference:

STOTEN 137091

To appear in:

Science of the Total Environment

Received date:

17 November 2019

Revised date:

30 January 2020

Accepted date:

1 February 2020

Please cite this article as: D.D. Sewu, H.N. Tran, G. Ohemeng-Boahen, et al., Facile magnetic biochar production route with new goethite nanoparticle precursor, Science of the Total Environment (2020), https://doi.org/10.1016/j.scitotenv.2020.137091

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© 2020 Published by Elsevier.

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Facile magnetic biochar production route with new goethite nanoparticle precursor

Divine Damertey Sewua, Hai Nguyen Tranb, Godfred Ohemeng-Boahena, and Seung

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Han Wooa*

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Department of Chemical and Biological Engineering, Hanbat National University,

Institute of Fundamental and Applied Sciences, Duy Tan University, Ho Chi Minh

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125 Dongseo-daero, Yuseong-gu, Daejeon 34158, Republic of Korea

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City, 700000, Vietnam

--------------------------------------------------------------------* Corresponding author. Tel: 82-42-821-1537; fax: 82-42-821-1593. E-mail address: [email protected] (S. H. Woo).

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Abstract This study fabricated a green and novel magnetic biochar via the co-pyrolysis of firwood biomass pre-treated with 10% (w/w) of either solid- (admixing; G10BCA) or liquid-phase (impregnation; G10BCI) goethite mineral (α-FeOOH). Newly fabricated magnetic biochars were characterized by inductively coupled plasma-optical emission spectroscopy (ICP-OES), Brunauer-Emmett-Teller (BET) equipment, X-ray powder diffractometry (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), proximate and elemental analyzer, and vibrating sample magnetometry. The effects of magnetic precursor, iron loading, and aqua-treatments on recoverability, magnetic properties, and stability (resistance to α-FeOOH reconstructive

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crystallization/dissolution reactions) were found and compared to those of magnetic biochar derived from conventional ferric chloride precursors (F10BCI). Results confirmed a direct correlation

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between biochar yields and ash contents with iron loading, irrespective of the magnetic precursor

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type. Despite the higher total carbon content (83.6% (w/w)) and surface area (299 m2/g) of F10BCI, α-FeOOH proved to be more effective at yielding magnetic biochars with nanostructured surfaces,

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lower water extractable components (thus green; G10BCA = 0.21 mg/mL; G10BCI = 0.16 mg/mL), higher magnetic saturation (G10BCA = 10.0 emu/g; G10BCI = 20.8 emu/g), and ferromagnetic

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susceptibility as well as recoverability. Furthermore, α-FeOOH was undetected on the surface of G10BCA, post-aqua-treatments (over 30 days), and this demonstrated its stability in the face of

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demagnetization via α-FeOOH reformation reactions. Consequently, this study showed that admixing solid-phase α-FeOOH (10% (w/w)) with firwood biomass offers a green, facile, and

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efficient way to thermochemically produce magnetic biochar. It has superb stability to α-FeOOH reconstructive crystallization/dissolution reactions in aqua media, green attributes, good magnetic

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properties, and great application potential in many areas of the economy. Keywords: Aqua-stability; green synthesis; goethite nanoparticle; magnetic biochar; co-pyrolysis.

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1. Introduction Biochar, which is a solid carbonaceous product of biomass pyrolysis, has certain economic advantages (for example, renewable waste biomass feedstock), carbon sequestration potential (carbon sink) and many applications in various parts of the economy (Boakye et al., 2016; Lehmann, 2007). One such area is water remediation, primarily because of its abundant and diverse surface functional groups, surface area, and porosity (Sewu et al., 2019; Zhang et al., 2017). Notwithstanding this, its application in real systems has proven time-consuming and somewhat

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expensive owing to separation costs, especially for powdered grades (Son et al., 2018b). As such,

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the production of magnetic biochar has attracted much attention as the solution because it offers a

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simpler, easier, more effective separation route than some traditional approaches like filtration (Yi et al., 2019). A wide array of sustainable, green and economically viable biomass resources—such as

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corn stalk, (Sun et al., 2015) peanut hull (Han et al., 2016), and firwood (Karunanayake et al.,

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2017)—have been successfully explored for producing magnetic biochar.

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In general, magnetic biochar is generally produced through three common methods (i.e., coprecipitation, calcination, and pyrolysis) employing different magnetic precursors (Thines et al.,

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2017) applied either pre- or post-biochar production (Tan et al., 2016). Although the magnetic

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precursors can be sourced from transition metals and their compounds, iron (Fe)-containing precursors are the preferred choice because they are innocuous and cost-effective (Tomul et al., 2019; Yi et al., 2019). Interestingly, of the Fe-containing precursors, prior and current published studies that utilize chloride or sulfate salts of Fe2+ and Fe3+ ions are the most common (data from the “Web of Science” database—summarized in Table S1). This raises environmental concerns since these salts may well have toxic properties. For instance, in the co-precipitation method, a reaction between Fe2+ and Fe3+ [1:2 (mole basis)] and a base (NaOH or NH4OH: which has a toxic character) is required (Baig et al., 2014; Yu et al., 2013). Furthermore, when the magnetic precursors are applied to pre-biochar production by impregnation/coating (63%; Table S1), then magnetization, which is achieved via a strongly

Journal Pre-proof reducing environment provided by hydrogen (H2) and carbon monoxide (CO) gases during the biomass pyrolysis in the presence of ferric chloride (FeCl36H2O) (Zhang et al., 2013), can potentially release chlorine gas and gaseous hydrochloric acid. These inadvertently flaunt the 12 principles of green chemistry proposed by Anastas and Warner (2012). A few years ago, Wang et al (2015) utilized a non-sulfate/chloride Fe-containing precursor, hematite (α-Fe2O3) to solve this environmental issue. However, as in all other instances, an additional drying step is required prior to the pyrolysis stage, since modification is achieved with liquid-phase precursors, making it an

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expensive process. To curb these problems, the application of a solid-phase precursor (goethite; α-

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FeOOH) in-situ in the biomass in magnetic biochar production is proposed as a viable solution.

not been explored until now (0%; Table 1).

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Herein it is identified as the “admixing method” (also commonly known as dried mixing) which has

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It is currently well-known that the iron oxides, i.e. magnetite (Fe3O4) and maghemite (γ-

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Fe2O3) have magnetic properties (Shokrollahi, 2017). They are the principal products responsible for the eventual magnetization of biochars when Fe-containing precursors are used. However, of the

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iron oxides, α-FeOOH and α-Fe2O3 are the most thermodynamically stable at aerobic surface

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conditions and are non-magnetic (Patra et al., 2016; Schwertmann and Cornell, 2008). Therefore,

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the likelihood of iron oxides re-forming either α-FeOOH or α-Fe2O3 is highly favored when conditions such as high pH and water are available (He and Traina, 2007). This scenario inadvertently leads to a gradual demagnetization and it is of particular concern because generally, biochar exhibits high pH (Dai et al., 2017). Consequently, it may present the right conditions for the reconstructive crystallization/dissolution reaction of α-FeOOH and/or α-Fe2O3 when applied in wastewater setups. Attempts to stop the loss of magnetism have mostly been undertaken in the field of biomedical applications (Shokrollahi, 2017), but these cannot be applied to wastewater treatment strategies; loss in adsorbent properties is necessary for adsorption. For this reason, it is essential to devise a simpler, green and cost-effective approach to halting rapid α-FeOOH reformation reactions of magnetic biochar in aquatic and other destabilizing environments, especially when Fe-containing

Journal Pre-proof precursors are employed. As such, this research, aimed to produce a green magnetic biochar by employing a highly abundant and environmentally benign Fe-bearing mineral (i.e., α-FeOOH; 10% w/w), as a green alternative to FeCl36H2O, either impregnated into (G10BCI) or admixed (G10BCA) with waste firwood biomass—sustainable and renewable—in a facile pyrolysis route. A comparison with magnetic biochars from the commonly used FeCl36H2O precursor was done, and the effects of magnetic precursor types, Fe loading, and aqua-treatment on the recoverability, magnetic saturation

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(including the paramagnetic and ferromagnetic percentages), and stability of magnetic biochars

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were thoroughly investigated. Physicochemical properties, as well as water-extractable residues

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analysis, were also conducted on the magnetic biochars to ascertain their green attributes and

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potential application in other fields.

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2. Materials and methods 2.1. Materials

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The FeCl36H2O and α-FeOOH chemicals were obtained from Sigma-Aldrich Chemical

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Co., South Korea. Dried firwood flakes (1.5 cm × 1.5 cm) were obtained from a local market in Daejeon, South Korea, milled to within 300 to 710 µm sizes with the aid of a tube mill (IKA® Tube

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Mill, CS001, USA), oven-dried for 18 h at 105 ºC, and stored in dry air-tight glass jars in desiccators at room temperature.

2.2. Preparation of biochar The magnetic biochar production process is schematically depicted in Figure 1. Two different production routes (i.e., wet impregnation and dry mixing) were used to produce the magnetic biochars. Firstly, the conventional method of wet impregnation of the dry firwood biomass with liquid-phase precursors was undertaken using either α-FeOOH (Figure 1a) or FeCl36H2O (Figure 1b). Specifically, approximately 2.0 g of either α-FeOOH or FeCl36H2O was weighed into a 250 mL vial and dispersion or solution formed with 70.76 mL or 45 mL of deionized

Journal Pre-proof water, respectively, whilst stirring. The prepared solutions were transferred into a beaker to completely saturate 18 g of firwood biomass corresponding to 10% [(w (precursor)/w (mixture)) × 100%]. Both the α-FeOOH- or FeCl3-saturated mixtures were transferred into aluminum stainless steel trays and oven-dried overnight at 105 ºC. The effect of the Fe polymorph on the magnetic biochars was also explored by retaining the Fe content in each precursor at a constant 10% (w/w). Thus, instead, 5.71 g of α-FeOOH and 9.68 g of FeCl36H2O harboring 2 g of Fe each, were measured into separate vials and filled with specific volumes of deionized water to make up the

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saturation ratio of 0.25:1 and 0.4:1, respectively [firwood: precursor solution (w/v)]. In the second

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new method (Figure 1a), however, solid-phase α-FeOOH was thoroughly mixed with firwood biomass via admixing (also known as dry mixing or physical mixing), using the same mixing

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conditions [10% (w/w)] as described earlier for the impregnation step, yet omitting the additional

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drying step. As controls, the pyrolyzed α-FeOOH (PG) nanoparticle and non-magnetic biochar

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(prepared from direct pyrolysis of firwood biomass; BC) were produced employing the aforementioned pyrolysis conditions (Figure 1c and d, respectively).

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A fixed bed pyrolysis reactor (Figure 2a) was then charged with the appropriate pre-treated

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sample (Sewu et al., 2017) and pyrolyzed for a sustained 1 h period at 500 ºC using a heating rate of

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10 ºC/min under N2 flow (rate of 200 mL/min). The obtained magnetic biochars were allowed to cool to room temperature within 180 min under N2 flow conditions. To compute the product yields, condensable volatiles (bio-oil) were condensed using a thermostat circulator, collected and weighed together with the biochars. The obtained biochars were sieved to within a 300–710 µm size range, collected, and stored in air-tight borosilicate glass vials. Magnetic biochars were labeled as G10BCI, F10BCI, FFe10BCI, and GFe10BCI representing biochar produced from firwood biomass impregnated with 10% (w/w) each of α-FeOOH, FeCl36H2O, Fe from FeCl36H2O, and Fe from αFeOOH, respectively. The magnetic biochar from the admixture of the solid-phase α-FeOOH and firwood biomass was also labeled as G10BCA (Figure 1).

Journal Pre-proof 2.3. Characterization of pristine and magnetic biochars The pristine and magnetic biochars were characterized for Brunauer-Emmett-Teller (BET) surface area, pore volume, and average pore diameters using N2 at 77 K as the adsorbate after having degassed the biochars for 6 h at 200 ºC (Micromeritics Tri StarTM II 3020, USA). Total carbon, hydrogen, and nitrogen in the biochars were ascertained via a CHN/O elemental analyzer on a dry basis (Thermo Flash 2000, UK). The different surface morphologies of the biochars were determined by a scanning electron microscopy (SEM) instrument (Joel Ltd, JSM-6390, USA) fitted

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with energy dispersive spectroscopy (EDS) for surface elemental analysis estimations (Oxford,

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ISIS, USA). Inductively coupled plasma-optical emission spectroscopy (ICP-OES) was employed

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for the qualitative and quantitative elemental analysis of biochars (PerkinElmer, 5300DV, USA). Xray powder diffractometer (XRD) served to analyze the crystalline phases of iron oxides present in

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the biochars (Rigaku, D/MAX 2500H, USA). To ascertain the magnetic saturation of these

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magnetic biochars, a hysteresis loop analysis of four selected samples (i.e., G10BCI, G10BCA, F10BCI, and PG) was done using a magnetometer (SQUID-VSM, QM00384) at room temperature

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with an applied field of –70 to +70 kOe. Proximate analysis was performed following the American

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Society for Testing and Materials standard (ASTM D1762-84) Chemical Analysis of Wood

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Charcoal with biochar-specific modification as reported by Enders et al. (2012) and Joseph et al. (2012). The pH of the biochars was determined by adhering to the International Biochar Initiative (IBI) Biochar Standards Version 2.0 (Test Category A test methods); the experiment was executed in two replicates. The water-extractable residues were determined gravimetrically. Specifically, 0.2 g of biochar and 10 mL of deionized water were mixed in a 20 mL vial and shaken at 30 ºC for 24 h at 150 rpm. The supernatant solution was filtered and transferred onto Al trays, dried at 105 ºC for 24 h, and the difference in the weight before and after drying was determined and recorded.

2.4. Magnetism analysis To analyze the paramagnetic and ferromagnetic components of the magnetic biochars, the Honda-Owen method was employed as shown here in Equation 1.

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x  x p  xd 

Ms M  x  s H H

(1)

where x is the mass magnetic susceptibility in a relatively low magnetic field; xp and xd are paramagnetic and diamagnetic specific susceptibilities, respectively; 𝑥∞ is the specific magnetic susceptibility extrapolated to the infinite magnetic field and is characteristic of a mineral; and Ms is the saturation magnetization of the ferromagnetic component of the mixture. This indicates the degree of ferromagnetism and it varies between different samples and size fractions.

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To construct the Honda-Owen plot, the magnetization versus magnetic field curve must

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show evidence of both paramagnetic and ferromagnetic components. Separating the two aspects of

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magnetization requires plotting the magnetic susceptibility, 𝑥 against the reciprocal of the magnetic field in the dM/dH region, which yields a linear relationship with the y-intercept being the magnetic

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susceptibility of the paramagnetic component. Ferromagnetic susceptibility is subsequently the

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difference between total magnetic susceptibility and paramagnetic susceptibility (Wu et al., 2012).

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2.5. Aqua-treatment effect on the properties of the magnetic biochars The magnetic biochars were subjected to different aqua-treatment ratios from 1:20 to 1:50

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(w:v) in order to determine the aqua-stability (resistance to α-FeOOH reformation reactions) and to

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effectively compare the known method of using FeCl3 as the magnetic precursor in magnetic biochar production to the novel approach (admixing) of utilizing solid-phase α-FeOOH as the magnetic precursor. The experiment was run for a period ranging from 1–30 days. Color changes of biochars in the aqua-environment were monitored; indirect observation of α-FeOOH reformation. Samples after 24 h were taken, dried at 105 ºC for 24 h, and analyzed qualitatively for α-FeOOH with XRD and using the SEM technique for any changes in the morphological surface.

2.6. Recovery of magnetic biochar In order to translate the magnetism analysis into a practical application, the efficiency of recovering the magnetic biochars in an aqua-environment was checked following the approach

Journal Pre-proof employed by Son et al (2018a) with modifications. More specifically, 30 mg of magnetic biochar (Wa) was measured into an already weighed (Wb) 20 mL glass vial after pre-heating at 105 ºC. Exactly 9 mL of deionized water was also measured and transferred into the glass vial corresponding to a ratio of 1:300 (w/v). The vial was placed in a shaking incubator for 1 h at 150 rpm. After shaking, a permanent magnet with a strength of 3.155 kOe was attached to the vial for recovery purposes, and the supernatant mixture was discarded. The recovered biochar was dried for 24 h at 105 ºC, and the weight (Wc) was determined. The percentage of recovery efficiency (R) was

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Wc  Wb  100 % Wa

(2)

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R

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then computed using the following equation:

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3. Results and discussion

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3.1. Production and yield of pristine and magnetic biochar samples The integrity of the pyrolysis reactor after the production of the magnetic biochars is shown

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in Figure 2b. The production of the biochars commonly yielded various dark colors in the brown

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range in the interior of the reactors. These general observations held true when α-FeOOH was employed to yield the magnetic biochars—G10BCA, G10BCI, and GFe10BCI—irrespective of the

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amount of α-FeOOH. However, utilizing the FeCl3 magnetic precursor generated white-crystalline deposits (or residuals) lining the interior of the reactor and reactor cap, thus heightening potential fouling tendencies which could narrow the aperture size (Figure 2b). The effect of magnetic precursor (α-FeOOH or FeCl3) on the yield of the main product of pyrolysis is presented in Table 2. Generally, magnetic biochar yields improved whereas bio-oil and biogas yields declined for all production routes employed when compared to the pristine, nonmagnetic biochar (BC), except for G10BCA, which had a higher bio-oil yield. Increasing the Fe content in both magnetic precursors to 10% (w/w) positively and negatively affected the biochar [(35.1% to 37.7% for FFe10BCI and 27.6% to 34.2% for GFe10BCI) and bio-oil yields (26.2% to

Journal Pre-proof 24.0% FFe10BCI and 28.6% to 20.7% GFe10BCI)], respectively. This suggests that less waste was being generated and therefore a commendably green process. More biochar was obtained when firwood biomass was treated with liquid-phase α-FeOOH (G10BCI) via impregnation than with solid-phase α-FeOOH (G10BCA) via admixing. As well, smaller biochar yields occurred when αFeOOH was employed (G10BCI = 27.6%) in comparison to FeCl3 precursors (F10BCI = 35.1%) in all cases, contrary to what was expected. A plausible explanation could be the C:O reaction between carbon in biomass and oxygen in α-FeOOH as opposed to none in FeCl3 (water loss via

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dehydration). As such a smaller amount of carbon remained which led to a smaller biochar yield

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after the pyrolysis process for α-FeOOH than for FeCl3-derived magnetic biochars.

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3.2. Characterization of pristine and magnetic biochar

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The proximate and elemental analyses data regarding the pristine and magnetic biochars are presented in Table 3. Ash and fixed carbon contents generally increased and decreased,

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respectively, for the magnetic biochars in comparison to the pristine biochar. Fixed carbon contents

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were lower in magnetic biochars derived from α-FeOOH precursors (G10BCI = 40.5% and GFe10BCI = 33.8%) than from FeCl3 precursors (F10BCI = 65.0% and FFe10BCI = 51.8%). A

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reverse trend, however, was observed for the ash contents which instead rose in magnetic biochars

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when α-FeOOH precursors were used by a factor double that of FeCl3 precursors except for G10BCA (15.4%) which was in the neighborhood of F10BCI (11.3%). Expectedly, the increase in the Fe contents to 10% (w/w) in the respective magnetic precursors led to a decrease in the fixed carbon content of the magnetic biochars. Fe is inorganic and will consequently appear as part of the ash content. α-FeOOH has more Fe than FeCl3 per unit mass and as such by extension will yield more ash content than FeCl3 when used as a precursor in magnetic biochar production. This explains the observed results for G10BCA and F10BCI. The above explanation, however, did not hold true on the basis of equal Fe contents in the precursors—GFe10BCI and FFe10BCI. A plausible reason could be that Fe in the presence of chlorides was easily vaporized (supported by the ICP data of white residues in Supplementary Table S2). As a result, the ash contents were different although

Journal Pre-proof the Fe contents used in the GFe10BCI and FFe10BCI were the same. Total carbon content also followed the same trend as fixed carbon content in the following decreasing order: BC > F10BCI > G10BCA > FFe10BCI > G10BCI > GFe10BCI. This confirms the previous explanation on the yield data. All magnetic biochars indicated a decrease in carbon, hydrogen, oxygen, and nitrogen except for F10BC I which increased oxygen content from 1.69% to 2.17% in the pristine biochar. G10BCA was comparable to F10BCI but exhibited higher elemental compositions of carbon (82.9%), hydrogen (0.33%), oxygen (1.55%), and nitrogen (0.31%) when

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compared to G10BCI (C = 72.3%, H = 0.27%, O = 1.17%, and N = 0.24%). These figures suggest

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that pyrolyzing admixed α-FeOOH (10%) and firwood biomass can: firstly, yield magnetic biochar

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(G10BCA) with better carbon sequestration potential; and secondly, preserve the hydrogen, oxygen

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and nitrogen contents than when firwood biomass was impregnated with α-FeOOH (G10BCI). The water-extractable species and textural properties of the pristine and magnetic biochars are

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summarized in Tables 3 and 4, respectively. Table 3 shows that the water-extractable species were

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the highest for F10BCI (0.84 mg/mL) and lowest for G10BCI (0.16 mg/mL), which is contrary to what was expected with comparable values for BC (0.25 mg/mL) and G10BC A (0.21 mg/mL).

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Likely reasons are the highly water-soluble chloride salts and higher surface area and porosity

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present in F10BCI (Table 4 and Figure S2) compared to G10BCI. Thus, more water can access the exposed surface areas and pores, and thus dissolve more water-soluble components. It is suggested that for application in aqueous systems, F10BCI has a greater propensity to release/leach undesirables into receiving systems and it is deemed to be less green in comparison to G10BCI and G10BCA. Furthermore, significant improvements in the surface area, pore volumes, and porosity were observed for all magnetic biochars as well as pH decreases when compared with BC (Table 4). Particularly, using FeCl3 precursors led to an improvement in the textural properties—surface area and porosity (F10BCI = 299 m2/g and 89.4%, respectively)—than with α-FeOOH precursors (G10BCA = 181 m2/g and 88.4%, respectively; G10BCI = 209 m2/g and 87.8%, respectively). Also, Fe content increment to 10% (w/w) for FeCl3 resulted in a positive effect on surface area (299 to

Journal Pre-proof 442 m2/g) but declined slightly for porosity (89.4 to 87.6%). A negative effect on surface area was observed for α-FeOOH precursors (209 to 173 m2/g) with more Fe loading; porosity overall remained mostly unchanged (87.8 to 87.4%). Pore sizes also remained unaffected at 2.0 nm, irrespective of the Fe amount for FeCl3 precursors but they did increase from 3.09 nm (G10BCI) to 3.56 nm (GFe10BCI) with α-FeOOH precursors. Figure 3 illustrates the SEM images at high magnifications with generally observed, nanosized particles on magnetic biochars from α-FeOOH precursors; increased with Fe loading—

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G10BCA, G10BCI, and GFe10BCI. The BC sample revealed a smoother and cleaner surface when

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compared with the magnetic biochars. These biochars’ rough surfaces, characterized by numerous

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lumps or projections, were identified as harboring Fe whereas the plain regions had none. To elucidate the possible role of magnetic precursors in enhancing the surface properties, Fe maps were

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taken (Figure S3). Fe maps turned up no results for BC (not shown). However, Fe distributions

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were uniform for all magnetic biochars except for FFe10BC I. Noticeably, the concentrated Fe regions in FFe10BCI surrounded the pores. This suggests the likely role of FeCl3 increment in

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creating more pores via catalyzing carbon decomposition reactions on magnetic biochar (supported

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by surface area and pore volume data in Table 4). Nonetheless, the uniform Fe distributions, G10BCA, G10BCI, and GFe10BCI displayed obstructions of pores to varying degrees, and they

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were intense for GFe10BCI (hence the smallest surface area of 173 m2/g; Table 4). Again, it was observed that the pores of G10BCA were devoid of Fe particles, which suggested that only an external surface interaction phenomenon (coating) occurred between the solid-phase α-FeOOH and firwood biomass in fabricating the magnetic biochar as expected. The XRD data concerning the pristine non-magnetic and magnetic biochars are shown in Figure 4a. As anticipated, no characteristic peak for iron oxide was observed on BC. The magnetic biochars, however, displayed peaks for Fe but none for iron oxide. The control sample, PG, turned up peaks for mainly maghemite (γ-Fe2O3) magnetics and possible (SiO2) quarts. A plausible explanation is the possible interferences experienced by using XRD on non-crystalline and mixed

Journal Pre-proof samples since biochar is largely amorphous (Liu et al., 2015). As such, although with a magnetic attribute (hysteresis loop analysis), the crystalline iron oxide content is likely to make up less than 10% of each magnetic biochar (Table 5 of ICP data) and this explains the obtained results. The qualitative and quantitative findings for the metallic components from ICP-OES of the pristine and magnetic biochars are shown in Table 5. Fe contents revealed the following decreasing order: GFe10BCI, FFe10BCI, G10BCI, F10BCI, G10BCA, and BC. However, based on the magnetic precursor and production method, a different trend where F10BCI and G10BCA were reversed was

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expected. Based on the mass of magnetic precursors being the same, Fe content should be higher in

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G10BCA than F10BCI. Such contrary results may thus be the outcome of fewer carbon- α-FeOOH

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interactions for G10BCA. Potassium (K) detection was null for magnetic biochar derived from FeCl3 precursors—F10BCI and FFe10BCI, but present in increasing amounts with increments in α-

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FeOOH (G10BCI = 6.110 g/kg and GFe10BCI = 7.749 g/kg). This implied that magnetic biochar

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production of F10BCI and FFe10BCI facilitated the release of K, which was otherwise present in firwood biomass. It is supported by ICP data on white deposits (3.56 g/kg of K) in the

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supplementary information (Table S2). α-FeOOH, however, suppressed K release in firwood

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biomass during pyrolysis, thus entrapping and potentially enhancing availability of target adsorbates

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in G10BCA, G10BCI, and GFe10BCI for ion exchanges. Apart from this, chromium (Cr) concentration–toxic metal (Yedjou et al., 2012) was respectively, null, little (0.11-0.19 g/kg), and high (0.82-11.6 g/kg) for production method employing solid-phase α-FeOOH, liquid-phase αFeOOH, and liquid-phase FeCl3 precursors. The aforementioned results suggest that admixing route with solid-phase α-FeOOH is the greenest in that it eliminates toxic Cr in the product and replaces FeCl3 precursors and its toxic tendencies. It can therefore better protect against environmental degradation and potentially prevent secondary pollution when employed in water or soil remediation applications.

3.3. Magnetic properties of magnetic biochar The extent of magnetization of the magnetic biochars was evaluated on three (3) magnetic

Journal Pre-proof biochars—F10BCI, G10BCA, and G10BCI—as well as on PG as the control, with the magnetic field ranging from -70 to +70 kOe (complete cycle) at room temperature. Figure 5a shows that the saturation magnetization increased in the order: F10BC I < G10BCA < G10BCI < PG. Interestingly, G10BCA (10.01 emu/g) indicated a slightly higher magnetic saturation of than F10BCI (9.42 emu/g) even with lower Fe content (Table 5). Using only 10% (w/w) α-FeOOH, G10BCI exhibited a magnetic saturation value of 20.8 emu/g, relatively similar to that of PG (24.5 emu/g) which had no biomass impurities. Subsequently, this indicated the advantage of using α-FeOOH as a magnetic

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precursor instead of the conventional use of FeCl3.

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Focusing on the smaller magnetic field range within –1.3 to +1.3 kOe revealed both

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paramagnetic and ferromagnetic behaviors (data not shown). The hysteresis was in an order similar to that of the saturation magnetization. The Honda-Owen plot (Figure S4) was thus at a field of 103

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kA/m. The plots highlighted extremely high R2 values (> 0.98) close to linearity for all magnetic

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biochars, indicating the appropriateness of applying the Honda-Owen plot. Magnetic susceptibility is loosely defined as the measure of the extent to which a substance becomes magnetized when in

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an external magnetic field. Paramagnetic materials lose their magnetic properties when the

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externally applied magnetic field is removed whilst ferromagnetic materials exhibit properties to the

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contrary (Morrish, 2001; Skomski, 2008). From the Honda-Owen plot, the total magnetic susceptibilities, as well as the paramagnetic and ferromagnetic percentages, were computed and are shown in Table 6. The total magnetic susceptibilities at 103 kA/m also revealed trends similar to the hysteresis loop data with ferromagnetic and paramagnetic behaviors increasing and decreasing, respectively, in that same order. Paramagnetic behavior may be more desirable in adsorption applications where easier adsorbent separation for efficient recovery for regenerative/reactivation processes and reuse is prioritized. The removal performances of the biochars in an aqua-environment are well captured in Figure 5b. It is notable that with high magnetic precursor amounts [harboring 10% (w/w) Fe], extremely high removal performances were seen in both FFe10BCI and GFe10BCI. G10BCI

Journal Pre-proof although harboring 10% (w/w) α-FeOOH precursor had (when inspected visually) comparable removal performances to FFe10BCI and GFe10BCI. G10BCA outperformed F10BCI with satisfactory removal efficiency (visual) which is consistent with the aforementioned trend for magnetization. It proves that environmentally benign α-FeOOH is a better magnetic precursor than FeCl3.

3.4. Aqua-treatment effect on the properties of magnetic biochars To determine the stability of the magnetic biochars with reference to retaining the magnetic

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properties, aqua-treatment was employed. The XRD data was taken for the resulting aqua-treatment

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magnetic biochars and are depicted in Figure 4b. After aqua-treatment, new compounds appeared

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on both pristine and magnetic biochars. F10BCI easily converted to α-FeOOH which is

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antiferromagnetic (Zhou et al., 2017). G10BCI also showed α-FeOOH and akaganeite (β-FeOOH) reformation but to a lesser extent. Only G10BCA had no α-FeOOH reformation occurring so it

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remained very stable and is suitable for application in adsorption setups.

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To elucidate the effect of aqua-treatments on the morphology of pristine and magnetic biochars, SEM images (Figure S5) were taken. It is evident that aqua-treatment resulted in the

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development of more pores with thinner walls for BC. However, morphological changes in the

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aqua-treated magnetic biochars (i.e., G10BCA and G10BCI) were indistinct from the as-produced magnetic biochars except for F10BCI which showed distinct pores (validated by the waterextractable residues’ trend; Table 3). It suggests the likelihood of higher bond strength between Fe or iron oxides on G10BCI and G10BCA, than for F10BCI (supported by ash content in Table 3). EDS (data not shown) also proved the near constancy of the surface elemental distribution for the aqua-treated magnetic biochars. Figure S6 shows the magnetic biochars’ change in color, postaqua-treatment, over a 24 h period. Evidently, all magnetic biochars remained black (typical of biochars) except for F10BCI and FFe10BCI. Both F10BCI and FFe10BCI exhibited a yellow coloration (characteristics of α-FeOOH) (de Faria and Lopes, 2007) with the former being more intense and the latter being less intense but more aggregated. However, for a period of 3 months

Journal Pre-proof (data not shown), F10BCI and FFe10BCI both showed extremely intense yellow coloration with G10BCA remaining visibly unchanged. Nonetheless, G10BCI and GFe10BCI betrayed a light yellowish coloration after the elongated 3-month aqua-treatment test period. These changes in color were attributed to the reconstructive crystallization/dissolution reaction of α-FeOOH (He and Traina, 2007). The data proved that G10BCA was the most resistant to α-FeOOH reformation reactions and it was the most stable, followed by G10BCI and GFe10BCI. Provided here is a very simple but cost-effective admixing approach involving the pyrolysis of biomass treated with solid-

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phase α-FeOOH, 10% (w/w). It produces magnetic biochar of superb stability in aquatic and high

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pH environments (Table 3) with a correspondingly green or environmentally friendly potential.

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3.5. Recovery efficiency analysis

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The recovery of the magnetic biochars in water environment was investigated on the asprepared and aqua-treated magnetic biochars for three consecutive cycles; the results are shown in

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Figure S6. Generally, all magnetic biochars exhibited higher recovery efficiencies at all cycles

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when aqua-treated, except for G10BCI. Only the recovery efficiency for the as-produced magnetic biochar in the first cycle followed the same trend for magnetic saturation (Figure 5a). Recovery

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efficiency loss was greatest in G10BCA for both as-produced (5.5% and 4.3%) and aqua-treated

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cases (7.0% and 5.0 %) in the second and third cycles, respectively. Recoverability loss may depend on: firstly, the higher water-extractable amounts; secondly, decreased or loss in magnetic property due to α-FeOOH reformation reactions (non-magnetic); and thirdly, wash-off of magnetic species on the surface of the magnetic biochar. Aqua-treated biochars were less water-extractable and thus retained more of the initial mass when recovered in comparison to the as-produced biochars. The discrepancy in G10BCI may be due to the significantly low water-extractable contents (0.16 mg/mL), so they did not proportionately affect the recovery efficiency. It was close to the initial mass retained even after aqua-treatment. However, the greater loss in recovery efficiency for G10BCA in both cases was instead attributed to the potential wash-off of magnetic species from its surface [aqua-stability (Figure S6) and low water-extractability (0.21 mg/mL)]. These confirm that

Journal Pre-proof magnetic biochar production with 10% (w/w) liquid-phase α-FeOOH precursor was more effective at retaining its magnetic properties even after repeated recovery cycles.

3.6. Comparison of the magnetic biochars to others in the literature The performance of the magnetic biochars prepared in this study was compared in terms of textural properties, production costs, magnetic properties, and preservation after repeated application cycles and green attributes. Our results and those of other adsorbents examined in the literature are tabulated in Table S3. In terms of the textural properties, dual enhancements in the

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BET surface area with functionalization was observed for all magnetic biochars, irrespective of the

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production method, except for those produced via co-precipitation (Tian et al., 2011) which

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exhibited a significant decline (from 501 to 19.4 m2/g) at higher temperatures (700 ºC).

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Enhancements in the surface area reportedly led to good performance when deployed in environmental applications for contaminant immobilization (Devi and Saroha, 2014; Jung et al.,

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2016; Wang et al., 2018). This emphasizes the potential of applying the magnetic biochars

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developed in this study for environmental remediation. Referring to cost, it was apparent that the numerous stages of magnetic production (Devi and Saroha, 2014; Jung et al., 2016; Wang et al.,

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2018), higher pyrolysis temperatures (Jung et al., 2016; Tian et al., 2011; Wang et al., 2015), and

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longer pyrolysis times (Devi and Saroha, 2014; Tian et al., 2011) will render the magnetic biochars more costly than those produced in this study. The saturation magnetization (Ms) of the as-produced magnetic biochars (G10BCI = 20.83 emu/g and G10BCA = 10.01 emu/g) was higher than the magnetic γ-Fe2O3 biochar derived from pinewood sawdust (5.68 emu/g) (Wang et al., 2018), comparable to the magnetic Fe3O4 biochar (19.0 emu/g) and activated carbon (20.8 emu/g) (Shan et al., 2016), but lower than magnetic Fe3O4 biochar from marine macroalgae (26.79 emu/g) (Jung et al., 2016). In terms of the preservation of magnetic properties, researchers seldom reported this although it is very essential for the repeated or prolonged application of magnetic biochars in environmental remediation. However, a report by Devi and Saroha (2014) on the preservation of the magnetic properties via stability tests showed

Journal Pre-proof poor stability in aquatic and aerobic conditions for zerovalent iron (ZVI)-derived magnetic biochar. This demonstrated the superiority of the magnetic biochars produced in this study to those in other analyses concerning resistance to the loss of magnetic properties via α-FeOOH reformation reactions; thus, they are ideal for repeated and prolonged applications. In terms of the magnetic biochars’ green attributes, all production routes employed chemicals with toxic features except for the mechano-chemical (Shan et al., 2016) and impregnation/coating routes (Wang et al., 2015). For instance, the method of co-precipitation which utilized salt solutions of Fe2+ and Fe3+ precursors of

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1:2 ratio (mole basis) required a base addition (NaOH – has toxic tendencies) (Tian et al., 2011).

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Moreover, the hydrothermal method carbonizes biomass in-situ solutions of chloride/sulfate salts of Fe2+ and Fe3+ precursors at high pressures. However, low temperatures with an added pyrolysis step

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(Wang et al., 2018); generate waste filtrate due to the accompanying filtration step.

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The reduction approach predominantly utilized a reducing agent, NaBH4 in reaction with Fe

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precursors to produce the magnetic ZVI onto the biochar (Devi and Saroha, 2014). Furthermore, with the electro-modification approach the biomass was impregnated with metallic precursors

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generated by the cathode and anode reactions with NaCl electrolyte. It generates strong oxidizing

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HOCl and OCl- agents via the passage of electricity (Jung et al., 2016). All the aforementioned

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methods employed in the production of magnetic biochars shown in Table S3—except for Devi and Saroha (2014 and Shan et al. (2016—were conducted before the pyrolysis stage. As such, the potential for toxic products being generated such as gaseous hydrochloric acid and chlorine gas during the pyrolysis process renders it environmentally unfriendly. In contrast, the magnetic biochars produced in this present study required no toxic chemical usage and thus are sustainable, renewable, and green.

4. Conclusion Co-pyrolyzing firwood biomass pre-treated with 10% (w/w) of either solid- (G10BCA) or liquid-phase (G10BCI), α-FeOOH, successfully produced new magnetic biochars with comparable surface areas (181 m2/g and 209 m2/g, respectively) to that of conventional FeCl3-derived magnetic

Journal Pre-proof biochars (299 m2/g). Irrespective of the precursor used, there was a direct correlation between magnetic biochar yield and Fe loading. Only G10BCA and G10BCI exhibited nanostructured surfaces. α-FeOOH proved to be more effective at yielding magnetic biochars with lower water extractable components (thus green; G10BCA = 0.21 mg/mL; G10BCI = 0.16 mg/mL), higher magnetic saturation (G10BCA = 10.0 emu/g; G10BCI = 20.8 emu/g), and ferromagnetic susceptibility as well as recoverability. XRD results, post aqua-treatment (stability test) indicated no α-FeOOH detection on G10BCA. The above results showed that co-pyrolyzing firwood biomass

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with only 10% (w/w) solid-phase α-FeOOH offers a simple and effective way to dampen rapid α-

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FeOOH reformation reactions in aqua-media. It also helps to produce magnetic biochar with green attributes and good magnetic properties with a potential for sustainable and renewable

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environmental applications.

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Acknowledgements

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This work was supported by Basic Science Research Program through the National Research

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Foundation of Korea (NRF – South Korea) funded by the Ministry of Education, Science and

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References

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Technology (Grant no. NRF- 2016R1D1A1B03935962).

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Journal Pre-proof H. Morrish, pp. 696. ISBN 0-7803-6029-X. Wiley-VCH, January 2001. 2001: 696. Patra AK, Kundu SK, Bhaumik A, Kim D. Morphology evolution of single-crystalline hematite nan ocrystals: magnetically recoverable nanocatalysts for enhanced facet-driven photoredox acti vity. Nanoscale 2016; 8: 365-377. Schwertmann U, Cornell RM. Iron oxides in the laboratory: preparation and characterization: John Wiley & Sons, 2008. Sewu DD, Boakye P, Jung H, Woo SH. Synergistic dye adsorption by biochar from co-pyrolysis of s

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Skomski R. Simple models of magnetism: Oxford University Press on Demand, 2008. Son E-B, Poo K-M, Chang J-S, Chae K-J. Heavy metal removal from aqueous solutions using engin eered magnetic biochars derived from waste marine macro-algal biomass. Science of The To tal Environment 2018a; 615: 161-168. Son E-B, Poo K-M, Mohamed HO, Choi Y-J, Cho W-C, Chae K-J. A novel approach to developing a reusable marine macro-algae adsorbent with chitosan and ferric oxide for simultaneous effi cient heavy metal removal and easy magnetic separation. Bioresource Technology 2018b; 25 9: 381-387. Sun P, Hui C, Azim Khan R, Du J, Zhang Q, Zhao Y-H. Efficient removal of crystal violet using Fe

Journal Pre-proof 3O4-coated biochar: the role of the Fe3O4 nanoparticles and modeling study their adsorptio n behavior. Scientific Reports 2015; 5: 12638. Tan X-f, Liu Y-g, Gu Y-l, Xu Y, Zeng G-m, Hu X-j, et al. Biochar-based nano-composites for the de contamination of wastewater: a review. Bioresource technology 2016; 212: 318-333. Tian G, Geng J, Jin Y, Wang C, Li S, Chen Z, et al. Sorption of uranium(VI) using oxime-grafted or dered mesoporous carbon CMK-5. Journal of Hazardous Materials 2011; 190: 442-450. Tomul F, Arslan Y, Başoğlu FT, Babuçcuoğlu Y, Tran HN. Efficient removal of anti-inflammatory fr

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Journal Pre-proof on magnetic modified sugarcane bagasse prepared by two simple steps. Applied Surface Sci ence 2013; 268: 163-170. Zhang M, Gao B, Varnoosfaderani S, Hebard A, Yao Y, Inyang M. Preparation and characterization of a novel magnetic biochar for arsenic removal. Bioresource Technology 2013; 130: 457-46 2. Zhang T, Zhu X, Shi L, Li J, Li S, Lü J, et al. Efficient removal of lead from solution by celery-deri ved biochars rich in alkaline minerals. Bioresource Technology 2017; 235: 185-192.

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Journal Pre-proof Table 1. Scientometric data on magnetic biochar production routes (from the “Web of Science” da tabase; up to 29th May 2019). Production

Publication

Iron (chloride salt) precursors

(number)

(Occurrence; %)ɸ

Impregnation/coating

41

63.4

Co-precipitation

12

83.3

Electro-modification

1

0

Hydrothermal carbonization

5

20.0

Admixing (solid-phase)

0

Modification method

approach

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1. Pre-treatment

Oxidative hydrolysis/reduction

75.0

25

44.0

1

0

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Mechano-chemical

28

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Co-precipitation

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2. Post-treatment

0

Note: ɸBased on the total publications for each production route (type); keyword search: “magnetic

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biochar (with ‘and’/‘or’)” combinations; details on each production type can be found in the supplementary information [(Table S1a-f) alongside the graphical representation of the production

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routes compiled from literature (Figure S1)].

Journal Pre-proof Table 2. Yield data of the primary products of pyrolysis (wt.%). Biochar

Biooil

Biogasa

BC

21.8

32.9

45.3

F10BCI

35.1

26.2

38.7

FFe10BCI

37.7

24.0

38.3

G10BCA

26.4

33.4

40.2

G10BCI

27.6

28.6

43.8

GFe10BCI

34.2

20.7

45.0

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NOTE: aCalculated from the difference.

Journal Pre-proof Table 3. Physicochemical properties and water-extractable residues analysis of the magnetic and non-magnetic biochar samples

3. Other properties pH — Cext** mg/mL

Magnetic biochar (using FeCl3) F10BCI FFe10BCI

Magnetic biochar (α-FeOOH) G10BCA G10BCI GFe10BCI

3.51±0.53 23.6±2.94 11.1±0.26 65.0±2.68

4.15±1.2 21.3±4.30 26.9±0.69 51.7±3.61

2.51±0.5 24.9±0.33 15.4±0.55 59.6±0.88

4.32±0.5 30.8±2.17 28.7±0.41 40.4±1.76

4.13±0.35 16.7±3.13 49.6±0.17 33.7±2.97

84.03 0.39 0.44 1.69

83.57 0.35 0.22 2.17

75.28 0.29 0.16 1.37

82.93 0.33 0.31 1.55

72.25 0.27 0.24 1.17

59.3 0.18 0.18 1.11

12±0.04 0.25±0.04

9.02±0.01 0.84±0.09

5.79±0.05 —

10.6±0.08 0.21±0.09

7.86±0.06 0.16±0.02

9.85±0.25 —

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2.50±1.5 28.1±1.86 3.73±0.12 68.1±1.98

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1. Proximate analysis Moisture wt.% wt.% Volatiles wt.% Ash wt.% Fixed carbon* 2. Ultimate analysis C % % H % N % O

Biochar (BC)

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Unit

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NOTE: *By difference; and **Water-extractable components.

Journal Pre-proof Table 4. Textural property of the prepared biochar samples Total pore volume

(m /g)

3

(cm /g)

Average pore width (nm)

Porosity

2

BC

40.7±0.13

0.0197

1.94

85.6

F10BCI

299±1.59

0.1491

2.00

89.4

FFe10BCI

442±0.98

0.2182

2.00

87.6

G10BCA

181±0.81

0.1068

2.43

88.4

G10BCI

209±0.98

0.1617

3.09

87.8

GFe10BCI

173±0.26

0.1537

3.56

87.4

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BET surface area

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Note: BET surface area ± standard error.

(%)

Journal Pre-proof Table 5. Concentration of main elements (g/kg) in the prepared biochar samples (determined by the ICP-OES analysis)

Non-magnetic biochar

Magnetic biochar (using FeCl3)

Magnetic biochar (using α-FeOOH)

F10BCI

FFe10BCI

G10BCA

G10BCI

GFe10BCI

K

2.01

NA

NA

4.70

6.11

7.75

Si

—

—

—

17.1

NA

84.7

Al

NA

0.26

0.09

6.25

13.6

19.1

Ba

—

—

—

0.72

1.44

2.04

Ca

14.2

7.33

7.50

10.1

NA

6.45

Cr

0.18

0.82

11.6

—

0.19

0.11

Fe

7.24

71.7

188

64.9

144

202

Mg

1.40

0.09

0.05

1.83

NA

3.04

Mn

0.47

—

0.10

4.17

NA

11.3

Ni

0.45

0.46

1.25

—

0.18

0.40

P

NA

—

—

0.49

0.82

1.08

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(BC)

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NOTE: NA: not applicable for RSD (relative standard deviation) values above 6%.

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Journal Pre-proof Table 6. Magnetic susceptibility of paramagnetic and ferromagnetic percentages at a magnetic field of 103 kA/m Total magnetic susceptibility (10-5 × m3/kg)

Paramagnetic

Ferromagnetic

component (%)

component (%)

2.80

89.9

10.1

G10BCA

3.36

85.9

14.1

G10BCI

7.13

83.6

16.4

PG

14.9

44.3

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F10BCI

30

55.7

Journal Pre-proof Figure 1. Schematic representation of the magnetic biochar preparation route using different magn etic precursors (i.e., α-FeOOH and FeCl3 as modifiers) and some abbreviations used in this study. T he important effect of two mixing methods, which were admixing (or dried mixing) and impregnati on between firwood and goethite, on the properties of magnetic biochar were compared .Figure 2. Schematic representation of (a) the fixed bed pyrolysis setup and (b) the reactor befor e and after pyrolyzing the mixture of FeCl3 and firwood biomass Figure 3. SEM image of the as-prepared samples: (a) non-magnetic and (b)–(f) magnetic biochar s

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Figure 4. XRD data: new compounds formation or disappearance of existing ones (a) the raw bio

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char and (b) aqua treated biochar samples

Figure 5. (a) Magnetic hysteresis loop analysis for the magnetic biochars and pyrolyzed goethite;

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and (b) illustration of magnetic field effect on magnetic biochars in aqua-environment

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Figure 6. Recovery efficiency of the untreated and aqua-treated magnetic biochars in aqua enviro nment in the presence of an applied magnetic field of strength 3.155 kOe (average ± standard deviat

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Journal Pre-proof Declaration of competing interests ☒ The authors declare that they have no known competing financial interests or personal relationsh ips that could have appeared to influence the work reported in this paper.

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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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Journal Pre-proof Graphical abstract

Highlights 

Magnetic biochar developed from two magnetic precursors (conventional FeCl3 and new αFeOOH) Green magnetic biochar developed from firwood and magnetic goethite precursor



Goethite admixed or impregnated with firwood before pyrolysis at 500 °C



Nano-structured surfaces with high saturation magnetization (20.83 emu/g)



Production process: cleaner/greener than conventional FeCl3-based method

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

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Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6